The transmission of digital data at high speeds, e.g., 9600 bits per second, over band-limited transmission channels, such as telephone voice channels, requires precision control over impairments resulting from linear distortion, nonoptimal choice of receiver sampling instant and carried phase shift. Intersymbol interference is a consequence of linear distortion and nonoptimum sampling. Phase jitter and frequency offset are manifestations of varying carrier phase shift.
My copending joint application with K. H. Mueller, J. Salz and D. A. Spaulding, Ser. No. 437,978 filed on Jan. 30, 1974, now U.S. Pat. No. 3,878,468 granted Apr. 15, 1975, disclosed a passband linear feedforward equalizer followed by a data-decision-directed, phase-tracking demodulating carrier-wave arrangement in a QAM high-speed, data transmission system. The passband arrangement there disclosed provided for joint carrier recovery and equalization adaptive to the respective products and differences of actual equalized and estimated reference signals at passband line frequencies. For carrier recovery the correlation of the actual and estimated equalizer output signals furnished a phase control signal. Equalizer tap gains were updated under the control of the mean-square error difference between actual and estimated equalizer output signals. The estimated equalizer output was derived from the difference between the actual equalizer output and the quantization of that output after remodulation to the passband of the received line signal. No pilot tones were required for carrier-wave recover.
It is an object of this invention to improve passband equalizers employed in high-speed suppressed-carrier data transmission systems by jointly setting tap-gain adjustments and compensating for transmission-channel carrier phase shifts based on a common error criterion and without employing pilot tones.
It is another object of this invention to combine a passband feedforward transversal equalizer, a baseband feedback equalizer and a data-decision-directed demodulating carrier wave control arrangement, all adaptive to a common error criterion, to compensate for intersymbol interference, frequency offset and phase jitter.
It is a further object of this invention to track rapidly varying transmission-channel phase shift from the same common error signal that controls the slowly adapting tap-gain coefficients of the feedforward and feedback transversal equalizers.